System, apparatus and method for efficient optical signal amplification with system monitoring features

ABSTRACT

A system and method for efficient optical signal amplification with system monitoring features are provided. For example, an optical repeater may include two different 4-port thin-film gain flattening filters (TF-GFFs), which may be connected to provide a high-loss loop-back (HLLB) path in the optical repeater for system monitoring. The 4-port TF-GFF may have four different ports and may integrate the functionalities of a conventional GFF and a coupler into a single component, thereby increasing power efficiency of the optical repeater.

BACKGROUND OF THE INVENTION Field of the Invention

Embodiments of the present disclosure relate to the field of opticalcommunication systems. More particularly, the present disclosure relatesto a system, apparatus and method for efficient optical signalamplification with system monitoring features.

Discussion of Related Art

Long-haul optical communication systems, such as submarine opticalcommunication systems, generally suffer from signal attenuationresulting from a variety of factors, including scattering, absorption,and bending. To compensate for attenuation, these long-haul systems mayinclude a series of optical amplifiers or “repeaters” that are spacedalong the transmission path between a transmitter and a receiver. Therepeaters amplify the optical signal in a manner that allows reliabledetection at the receiver. Typically, multiple repeaters are positionedalong the transmission path depending on the length of the opticalcommunication system.

It may be important to monitor the working condition of a long-hauloptical communication system, such as detecting faults or breaks in thetransmission cable, detecting faulty optical repeaters, or detectingother problems with the system. Known monitoring techniques include theuse of various types of line monitoring equipment (LME) that maygenerate a test signal representing a pseudo random bit sequence, whichmay then be transmitted into the optical cable. The test signal isreturned to the line monitoring equipment through a high-loss loop-back(HLLB) passive coupling at various locations along the optical cable.The LME monitors the returned test signal and processes the test signalto obtain data representing the HLLB loop gain or changes in the gainimparted to the test signal from each of the coupling locations alongthe optical cable including within the optical repeaters. HLLBmonitoring may use loop gains or changes therein to characterize theoptical path or to detect changes in the optical paths which mayindicate a system fault.

With increasing capacity demand in these long-haul optical communicationsystems, the spatial and frequency densities of individual fiber optictransmission cables have been substantially increasing. This in-turnalso increases the power levels and overall power consumption of thesesystems. Delivering high power levels can present a significanttechnical and economic challenge, for example, in submarine opticalcommunication systems where the electrical power for the wet plantportion of the system must be transported along the cable. As a result,power efficiency may be an important consideration in overall systemdesign.

One known technique for reducing power consumption in the wet plantequipment is increasing the power conversion efficiency of the activecomponents within the optical repeaters, such as pump lasers,Erbium-doped fibers (EDFs), and the like. Apart from the activecomponents, however, the excess power loss of various passive componentsin the optical repeaters may also largely limit overall powerefficiency. For example, in an EDFA, the passive components may includea gain flattening filter (GFF), a band pass filter, an isolator, and acoupler, all of which may be essential in providing controllablerepeater gain. One known example of an EDFA repeater design thatintegrates HLLB features includes a fiber Bragg grating (FBG) GFF(hereinafter referred to as “FBG-GFF”), two isolators, and a coupler atthe output stage of the EDFA repeater. In addition to FBG-GFFs, anothertype of GFF that may be used in EDFAs is a thin-film GFF (hereinafterreferred to as “TF-GFF”). Typically, a TF-GFF may be composed of a pairof fiber collimators (which uses a micro-lens to transform the lightoutput from an optical fiber into a free-space collimated beam and viceversa) and a filter element, such as a substrate glass plate coated withmulti-layered dielectric thin films to realize a specific filterspectral shape between the collimators. A conventional TF-GFF is atwo-port device with input and output fibers. The collimated light beamfrom the input fiber is transmitted through the filter element and thencoupled into the output fiber by the second collimator.

Reducing partial or total power loss from one or more of theabove-described passive components (e.g., GFFs, FBG-GFFs, TF-GFFs, bandpass filters, isolators, couplers, etc.) in an optical repeater (e.g.,EDFA repeater) may produce a direct increase in power conversionefficiency within long-haul optical communication systems.

SUMMARY OF THE INVENTION

Embodiments of the present disclosure are directed to a system andmethod for efficient optical signal amplification with system monitoringfeatures. In one embodiment, a system may include an optical repeaterfor a fiber pair and one or more monitoring connections. The opticalrepeater may include a first 4-port thin-film gain flattening filters(TF-GFF) coupled to a first fiber and a second 4-port TF-GFF coupled toa second fiber, each 4-port TF-GFF having a first and secondtransmission port and a first and second reflection port. The opticalrepeater may also include an optical time-domain reflectometer (OTDR)filter that is connected to the first and second TF-GFFs. Moreover, themonitoring connections may include a first node and/or a second node,where the first node and/or the second node is configured to receive aline monitoring equipment (LME) channel signal for propagating the LMEchannel signal in the optical repeater for system monitoring.

In another embodiment, a method may include amplifying, via an opticalrepeater, a first signal input to a first fiber, and amplifying, via theoptical repeater, a second signal input to a second fiber. The methodalso includes receiving one or more line monitoring equipment (LME)channel signals from an LME and propagating the one or more LME channelsignals in the optical repeater. The method further includes providingthe propagated one or more LME channel signals to the LME for monitoringthe optical repeater. The optical repeater may include a first 4-portthin-film gain flattening filters (TF-GFF) coupled to a first fiber anda second 4-port TF-GFF coupled to a second fiber, where at least thefirst and second 4-port TF-GFFs are connected to form a high-lossloop-back (HLLB) path.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example 4-port thin-film gain flattening filter(TF-GFF) in accordance with one or more embodiments of the disclosure.

FIG. 2 illustrates an example graph of a reflection path insertion loss(IL) spectrum and a transmission path IL spectrum in accordance with oneor more embodiments of the disclosure.

FIG. 3A illustrates a schematic of an optical communication system inaccordance with one or more embodiments of the disclosure.

FIG. 3B illustrates a schematic of an example optical repeater inaccordance with one or more embodiments of the disclosure.

FIG. 4 illustrates a schematic of another example of an optical repeaterin accordance with one or more embodiments of the disclosure.

FIG. 5 illustrates a schematic of an example long-reach high-lossloop-back (HLLB) optical repeater in accordance with one or moreembodiments of the disclosure.

FIG. 6 illustrates a schematic of another example of a long-reach HLLBoptical repeater in accordance with one or more embodiments of thedisclosure.

FIG. 7 illustrates a schematic of an example double-pass HLLB opticalrepeater in accordance with one or more embodiments of the disclosure.

FIG. 8 illustrates a schematic of another example of a double-pass HLLBoptical repeater in accordance with one or more embodiments of thedisclosure.

DESCRIPTION OF EMBODIMENTS

The present invention is directed to a system, apparatus and method foramplifying optical signals using an optical repeater with high powerconversion efficiency. For example, the optical repeater may include anew and novel type of TF-GFF, a 4-port TF-GFF, which integrates thefunctionality of a conventional gain flattening filter and an opticalcoupler. This design of the gain flattening filter reduces overall powerloss by combining the functionalities of at least two passive componentsof the optical repeater while providing circuit integration that reducessystem and hardware related costs. Moreover, the optical repeater mayinclude various HLLB features for system monitoring.

As described above, optical repeaters of prior solutions includeseparate and different types of passive components, such as an FBG-basedGFF or a two port TF-GFF and a coupler which consume various levels ofpower. The one or more embodiments, examples, and/or aspects disclosedherein directed to a new and novel type of thin-film gain flatteningfilter, e.g., a 4-port TF-GFF, replaces at least the FBG-based GFF orthe two port TF-GFF and the coupler components of previous solutions,which increases overall power efficiency of a repeater while providingsystem monitoring capabilities, thereby overcoming the problems inherentin the previous solutions.

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention, however, may be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, like numbers refer to like elements throughout.

Referring to the drawings, FIG. 1 illustrates a 4-Port thin-film gainflattening filter (TF-GFF) 100. As shown, the 4-port TF-GFF includes adual fiber collimator 102, a dual fiber collimator 104, port 106, 108,110, 112, and a filter plate 114. Ports 106, 108, 110, and 112 may beoptical fibers terminated at their respective ends with fiber opticconnectors, which may be referred to as “fiber optic pigtails.” It maybe understood that the filter plate 114 may be positioned slightlynon-perpendicular relative to the direction of a collimated beam, forinstance, so that reflected light from the filter plate 114 is notcoupled back into the input fiber optic pigtail to ensure sufficientlyhigh return loss.

According to an embodiment, light input from port 106 may be partiallytransmitted through the filter plate 114 and traced to port 112, whilesome of the light may be reflected from the filter plate 114 and tracedto port 108 (where the traces are indicated by the single arrows). In asimilar manner, light input from port 108 may be partially transmittedthrough the filter plate 114 and traced to port 110 while simultaneouslybeing reflected from the filter plate 114 and traced to port 106 (wherethe traces are indicated by the double arrows).

In further embodiments, light input from the right side of the 4-portTF-GFF may trace and reflect similarly to light input from the left sideof the 4-port TF-GFF, as described above. Thus, light input from ports110 and 112 may be transmitted and traced to ports 108 and 106,respectively, and reflected from the filter plate 114 and traced toports 112 and 110, respectively (where the traces are shown by thetriple arrows for port 110 and the quadruple arrows for port 112).[Suggest remove this sentence and give further clarification in thesection below.]

FIG. 2 illustrates an exemplary graph 200 of a reflection path insertionloss (IL) spectrum 202 and a transmission path IL spectrum 204 of aTF-GFF (e.g., a 4-port TF-GFF) according to embodiments. For example,the reflection path IL spectrum 202 corresponds to or matches a specificErbium-doped fiber (EDF) GFF target spectrum, and the transmission ILspectrum is complementary to that EDF GFF target spectrum. A TF-GFFfilter may be designed or configured to have IL spectrum of either thetransmission path or the reflection path to match the target EDF gainflattening spectrum. This path, may be referred to as “GF path” And thecorresponding reflection path or transmission path may be referred to as“complementary path.” The selection of the filter design options, i.e.whether to use transmission path or reflection path as a GF path, may bebased on which approach can give smaller GFF shape errors and/or lowerloss as further described below.

Accordingly, the 4-port TF-GFF 100 incorporates the functionality of aconventional GFF through GF paths while providing additional signalfeedback through complementary paths or pigtails for system monitoringpurposes which would otherwise require a conventional coupler. As aresult, this new and novel type of GFF may be used as a GFF-couplerhybrid component, which allows lower optical attenuation than the sum ofthose two individual components, so as to achieve higher powerefficiency and space-cost reduction.

FIG. 3A illustrates an example bi-directional optical communicationsystem 301. As shown, a monitoring system may be implemented in thebi-directional optical communication system 301. The opticalcommunication system 301 may include terminals 303, 305 connected by twounidirectional optical paths 311, 321, which together form abi-directional optical path pair. Optical path 311 may transmitinformation in one direction from a transmitter 313 in the terminal 303to a receiver 315 in the terminal 305. Optical path 321 may transmitinformation in the other direction from a transmitter 325 in theterminal 305 to a receiver 323 in the terminal 303. With respect toterminal 303, the optical path 311 is an outbound path and the opticalpath 321 is an inbound path. The optical path 311 may include opticalfibers 317-1 to 317-n and optical amplifiers 319-1 to 319-n, and theoptical path 321 may include optical fibers 327-1 to 327-n and opticalamplifiers 329-1 to 329-n.

The optical path pair (e.g., optical paths 311, 321) may be configuredas a set of amplifier pairs 319-1 to 319-n and 329-1 to 329-n withinrepeaters 331-1 to 331-n connected by pairs of optical fibers 317-1 to317-n and 327-1 to 327-n, which may be included in an optical fibercable together with fibers supporting additional path pairs. Eachrepeater 331 may include a pair of amplifiers 319, 329 for each pathpair and may include additional amplifiers for additional path pairs.The optical amplifiers 319, 329 may utilize EDFAs or other rare earthdoped fiber amplifiers, Raman amplifiers or semiconductor opticalamplifiers. A coupling path 333-1 to 333-n may be coupled betweenoptical paths 311, 321, for example, in one or more of the repeaters331-1 to 331-n and may include, for example, one or more passive opticalcoupling components, as will be described in greater detail below.

Monitoring equipment, LME 341, may be located in one or both of theterminals 303, 305 to provide passive line monitoring for the opticalpaths 311, 321 using, for example, OTDR, COTDR or HLLB. LME 341 mayperform the signal generation, signal detection and processing functionsand may include transmitters, receivers, and processors configured toperform those functions. LME 341 may launch a monitoring or test signalinto optical path 311 (e.g., an outbound optical path). Coupling paths333-1 to 333-n may couple a sample of the monitoring or test signalpropagating in optical path 311 into the forward propagating directionof optical path 321 (e.g., an inbound optical path). LME 341 may thenreceive and measure the samples.

Although the coupling paths 333-1 to 333-n are shown as located witheach pair of amplifiers 319-1 to 319-n and 329-1 to 329-n, the couplingpaths may be located in other locations (e.g., outside of amplifierpairs) and may not be located in every amplifier pair. According to anexemplary embodiment, the coupling paths 333-1 to 333-n may be symmetricin operation, e.g., the function that describes the percent of opticalpower at each wavelength transferred from path 311 to path 321 by acoupling path 333-1 is the same as the function that describes thepercent of optical power at each wavelength transferred from path 321 topath 311 by the coupling path 333-1. Alternatively, one or more couplingpaths may not be symmetric and different coupling paths may havedifferent transfer functions. Coupling paths 333-1 to 333-n may also bedifferent for OTDR/COTDR monitoring and HLLB monitoring. A coupling pathfor OTDR/COTDR generally couples only reflected light (e.g., reflectedOTDR test signals) on the outbound path 311 into the inbound path 321,whereas a coupling path for HLLB couples monitoring signals transmittedon the outbound path 311 into the inbound path 321.

Although an exemplary embodiment of the optical communication system 301is shown and described, variations of the optical communication system301 are within the scope of the present disclosure. The opticalcommunication system 301 may include, for example, more optical pathpairs and more or fewer repeaters. Alternatively, the opticalcommunication system 301 may not include any optical amplifiers or mayinclude, instead of optical amplifiers, optical pump power sourcessuitable for implementing optical gain by Raman amplification withinoptical fibers connecting repeaters.

According to an exemplary method of monitoring optical communicationsystem 301, one or more OTDR test or probe signals may be transmitted(e.g., by LME 341) on the outbound optical path 311. As used herein,OTDR may generally refer to both standard OTDR and coherent OTDR(COTDR). Moreover, multiple OTDR test signals may be transmitted (e.g.,at different wavelengths) while loading the outbound optical path 311and/or the inbound optical path 321 to provide differential monitoring.OTDR test signals transmitted on outbound path 311 may be reflected byone or more reflecting elements in outbound optical path 311 and thereflected OTDR test signals may be coupled onto inbound optical path 321by one or more of the coupling paths 333-1 to 333-n.

It may be understood that the LME, or any other suitable systemmonitoring device, may be include at least one memory and one or moreprocessors (e.g., CPU, ASIC, FGPA, any conventional processor, etc.) toexecute instructions stored in memory. Moreover, the system monitoringfeature may be at least partially implemented as a program ofinstructions on a non-transitory computer readable storage mediumcapable of being read by a machine (e.g., LME, system monitoring device,any suitable computing device, etc.) capable of executing theinstructions. Thus, in examples, the LME may execute a program thatallows the LME to generate and transmit LME channel or test signals,which can be routed back to the LME from the HLLB of the opticalrepeater 302 and analyzed for system monitoring purposes.

FIG. 3B illustrates an exemplary optical repeater 331 n (renumbered as302 for ease of reference) implemented in a bi-directional opticalcommunication system 301. The optical repeater 302 may be configured ordesigned for a pair of optical fibers and incorporates two different4-port TF-GFFs that use transmission path IL to target the gainflattening spectrum in order to realize, for example, EDFA and HLLBfunctionalities, such as gain tilt monitoring, optical time-domainreflectometer (OTDR) capabilities, etc. As shown, at one side (e.g., the“east” side), the repeater 302 may include at least an optical isolator306, a wavelength-division multiplexer (WDM) 308, an EDF coil 310, anoptical isolator 312, a 4-port TF-GFF 314, and a dual fiber grating(DFG) 316. The DFG may be used in a system with two LME channels forgain tilt monitoring and may be replaced by a single channel fibergrating if only one LME channel is used in the system. A second side ofthe optical repeater 302 (e.g., the “west” side) may be similarlyarranged with similar components: at least an optical isolator 318, aWDM 320, an EDF coil 322, an optical isolator 324, a 4-port TF-GFF 326,and a DFG 328.

As will be further described below, an OTDR filter 330 may be arrangedbetween the east side and the west side of the optical repeater 302, forexample, disposed between the 4-port TF-GFFs 314 and 326. Moreover, anoptical pump unit (OPU), which may include multiple pump lasers, may becoupled to the WDMs 308 and 320. Although not shown, line monitoringequipment (LME) or other suitable types of system monitoring devices maybe connected to the optical repeater 302 for purposes of monitoring thesystem for component failure as described above with reference to FIG.3A. The LME may be connected to nodes 340 and 350, as shown, where LMEchannels or test signals may be input, transmitted, and/or received, forinstance, at these nodes.

The EDF coil 310 may output an amplified optical signal at a particularwavelength. During operation, the amplified signal output from the EDFcoil 310 may be flattened via a transmission path of the 4-port TF-GFF314, which may then be output to the “east out” port, and a residualamplified signal may be reflected by the 4-port TF-GFF 314.Additionally, LME channel signals from node 340 may be reflected by theDFG 316 and passed back through the 4-port TF-GFF 314. Both thereflected-residual amplified signal and the reflected LME channelsignals may be passed through the OTDR filter 330, where the signals maybe reflected by the 4-port TF-GFF 326 to the “west out” port. The OTDRfilter 330 may be designed to have minimum attenuation at specificwavelengths, e.g., wavelength of the reflected-residual amplifiedsignal, reflected LME channel wavelength, etc. To at least that end, theLME channel signals from the east side of the optical repeater 302 maybe looped back into the west side, thereby achieving conventional HLLBfunctionality. In a similar manner, the LME channel signals from thewest side may be looped back to the east side.

According to further examples, Rayleigh backscatter signals from theeast out port by the optical fiber may be reflected by the 4-port TF-GFF314 and passed through the OTDR filter 330, which may be reflected tothe west out port by the 4-port TF-GFF 326. It may be understood thatthe received Rayleigh backscatter signals at the west out port provideOTDR functionality, for example, as conventional HLLB OTDR function. Ininstances, because the reflectivity associated with the 4-port TF-GFF314 (or the 4-port TF-GFF 326) may not be high enough to limit the powerof the backscatter signal, the OTDR filter 330 may be configured tocompletely or partially suppress the in-band backscatter signal powerbased on the requirement of in-band OTDR functionality.

It may be understood that the various components shown in the opticalrepeater 302 may be connected, arranged, coupled, attached (whicheverdefinition applies) in any suitable manner to allow proper operation ofthe optical repeater and its functionalities.

FIG. 4 illustrates an optical repeater 400, which is an alternativeembodiment of the optical repeater 302 illustrated in FIG. 3B. As shown,the optical repeater 400 includes an optical isolator 406, a WDM 408, anEDF coil 410, an optical isolator 412, a 4-port TF-GFF 414, a DFG 416,an optical isolator 418, a WDM 420, an EDF coil 422, an optical isolator424, a 4-port TF-GFF 426, a DFG 428, an OTDR filter 430, thefunctionalities of which may be the same as the components of repeater302 of FIG. 3B. The design and configuration, however, of the opticalrepeater 400 is different in that the 4-port TF-GFFs 414 and 426 usereflection path IL (as opposed to transmission path IL used in FIG. 3B)to target the gain flattening spectrum.

As shown, signals from the reflection path of the 4-port TF-GFF 414 maybe output to the “east out” port of the optical repeater 400. Thecomplementary transmission path, or the transmission port, of the 4-portTF-GFF 414 may be used for LME channel signal loop-back from node 440 inthe optical repeater 400. Similarly, signals from the reflection path ofthe 4-port TF-GFF 426 may be output to the “west out” port, and thecomplementary transmission path, or the transmission port, may be usedfor LME channel signal loop-back from node 450.

FIG. 5 illustrates a long-reach HLLB optical repeater 500 according toone embodiment. The long-reach HLLB optical repeater 500 may include atleast an optical coupler (CPL) 506, an optical isolator 508, a WDM 510,an EDF coil 512, an optical isolator 514, a 4-port TF-GFF 516, a CPL518, and a DFG 520 arranged on the east side of the repeater 500, andfurther, may include at least a CPL 522, an optical isolator 524, a WDM526, an EDF coil 528, an optical isolator 530, a 4-port TF-GFF 532, aCPL 534, and a DFG 536 arranged on the west side. The long-reach HLLBoptical repeater 500 may also include an OTDR filter 538 that isarranged between the connection of the 4-port TF-GFFs 516 and 532, asshown. The long-reach HLLB optical repeater 500 may be configured suchthat the LME channel signals from node 540 at the east side are routedto both the west side input (via a separate fiber, cable, path, etc., asshown) and the west side output (via the path from the 4-port TF-GFF516, through the OTDR filter 538, and to the 4-port TF-GFF 532). Anidentical or similar configuration may be set up from the west side tothe east side, as illustrated.

Similar to the optical repeater 302 of FIG. 3B, the amplified signalsoutput from the EDF coils 512 and 528 may be flattened via thetransmission paths of the 4-port TF-GFFs 516 and 532, respectively.Advantageously, long-reach HLLB optical repeaters, such as the onedepicted in FIG. 5, may be used or implemented in optical communicationsystems having longer span lengths in order to have the test signal,e.g., LME channel signal, OTDR signal, etc. cover the full span.

FIG. 6 illustrates a long-reach HLLB optical repeater 600, which is analternative embodiment of the long-reach HLLB optical repeater 500 ofFIG. 5. As depicted, the long-reach HLLB optical repeater 600 mayinclude at least a CPL 606, an optical isolator 608, a WDM 610, an EDFcoil 612, an optical isolator 614, a 4-port TF-GFF 616, a CPL 618, a DFG620, a CPL 622, an optical isolator 624, a WDM 626, an EDF coil 628, anoptical isolator 630, a 4-port TF-GFF 632, a CPL 634, a DFG 636, and anOTDR filter 638.

While the components and the functionalities of the long-reach HLLBoptical repeater 600 is the same as the optical repeater 500 of FIG. 5,the long-reach HLLB optical repeater 600 is differently configured inthat the 4-port TF-GFFs 616 and 632 of the optical repeater 600 usereflection path IL (as opposed to transmission path IL) to target thegain flattening spectrum. Thus, the transmission port of the 4-portTF-GFF 616 may be used for LME channel signal loop-back from node 640 inthe optical repeater 400. And the same at node 650 for the 4-port TF-GFF632.

FIG. 7 illustrates a double-pass HLLB optical repeater 700 according toone embodiment. For example, the double-pass HLLB optical repeater 700includes, at the east side, a CPL 706, an optical isolator 708, a WDM710, an EDF coil 712, an optical isolator 714, a 4-port TF-GFF 716, aCPL 718, and a DFG 720. At the west side, the repeater 700 includes aCPL 722, an optical isolator 724, a WDM 726, an EDF coil 728, anisolator 730, a 4-port TF-GFF 732, a CPL 734, and a DFG 736. Moreover, aOTDR filter 738 is arranged between the east and west sides, e.g.,between the connections of the 4-port TF-GFFs 716 and 732.

As shown, LME channel signals from node 740 at the east side is routedback to the input of the repeater on the west side, e.g., at the “westin” port side. Similarly, the LME channel signals from node 750 at thewest side is routed back to the input on the east side, e.g., at the“east in” port side. Like the above transmission-IL-based examples, theamplified signals output from the EDF coils 712 and 728 may be flattenedvia the transmission paths of the 4-port TF-GFFs 716 and 732,respectively. Thus, the reflection ports of the 4-port TF-GFFs 716 and732 may be used for the LME channel signal loop-back and OTDR functions.

By way of example, the double-pass HLLB optical repeater 700 may be usedor implemented in optical communication systems to enhance the signatureof pump failure in the OPU that may be detected by the LME or othersuitable system monitoring devices, as the monitoring signals can bedouble amplified while passing through both EDF coils 712 and 728, whichshare the same OPU and are both sensitive to the failure therein.

FIG. 8 illustrates a double-pass HLLB optical repeater 800, which is analternative embodiment of the double-pass HLLB optical repeater shown inFIG. 7. The components of the double-pass HLLB optical repeater 800 (CPL806, optical isolator 808, WDM 810, EDF coil 812, optical isolator 814,4-port TF-GFF 816, CPL 818, DFG 820, CPL 822, optical isolator 824, WDM826, EDF coil 828, isolator 830, 4-port TF-GFF 832, CPL 834, DFG 836,and OTDR filter 838) and the functionalities thereof may be the same asthe repeater of FIG. 7, except that the 4-port TF-GFFs are configureddifferently in that the 4-port TF-GFFs 816 and 832 of the opticalrepeater 800 use reflection path IL to target the gain flatteningspectrum, and thus, the transmission ports of the TF-GFFs may be usedfor the LME channel signal loop-back.

Herein, novel and inventive apparatus and techniques for efficientoptical signal amplification with greater power efficiency and withsystem monitoring features are disclosed. The present disclosure is notto be limited in scope by the specific embodiments described herein.Indeed, other various embodiments of and modifications to the presentdisclosure, in addition to those described herein, will be apparent tothose of ordinary skill in the art from the foregoing description andaccompanying drawings.

Thus, such other embodiments and modifications are intended to fallwithin the scope of the present disclosure. Further, although thepresent disclosure has been described herein in the context of aparticular implementation in a particular environment for a particularpurpose, those of ordinary skill in the art will recognize that itsusefulness is not limited thereto and that the present disclosure may bebeneficially implemented in any number of environments for any number ofpurposes. Accordingly, the claims set forth below should be construed inview of the full breadth and spirit of the present disclosure asdescribed herein.

What is claimed is:
 1. A system for optical signal amplification,comprising: an optical repeater for a fiber pair comprising: a first4-port thin-film gain flattening filter (TF-GFF) coupled to a firstfiber, wherein the first 4-port TF-GFF has a first transmission port, asecond transmission port, a first reflection port, and a secondreflection port; a second 4-port TF-GFF coupled to a second fiber,wherein the second 4-port TF-GFF has a first transmission port, a secondtransmission port, a first reflection port, and a second reflectionport; an optical time-domain reflectometer (OTDR) filter having a firstside and a second side, wherein the first reflection port of the first4-port TF-GFF is coupled to the first side of the OTDR filter andwherein the first reflection port of the second 4-port TF-GFF is coupledto the second side of the OTDR filter; and one or more monitoringconnections comprising: a first node and/or a second node, wherein thefirst node and/or the second node is configured to receive a linemonitoring equipment (LME) channel signal for propagating the LMEchannel signal in the optical repeater for system monitoring.
 2. Thesystem of claim 1, wherein the optical repeater further comprises: afirst isolator and a second isolator; a first wavelength-divisionmultiplexer (WDM); and a first Erbium-doped fiber (EDF) coil, whereinthe first isolator, the second isolator, the first WDM, and the firstEDF coil are coupled to or arranged on the first fiber at a side of thefirst transmission port of the first 4-port TF-GFF, and wherein thefirst WDM and the first EDF coil are arranged between the first andsecond isolators.
 3. The system of claim 2, wherein the optical repeaterfurther comprises: a third isolator and a fourth isolator; a secondwavelength-division multiplexer (WDM); and a second Erbium-doped fiber(EDF) coil, wherein the third isolator, the fourth isolator, the secondWDM, and the second EDF coil are coupled to or arranged on the secondfiber at a side of the first transmission port of the second 4-portTF-GFF, and wherein the second WDM and the second EDF coil are arrangedbetween the third and fourth isolators.
 4. The system of claim 1,wherein the optical repeater further comprises: a first dual fibergrating (DFG) and a second DFG, and wherein the first DFG is coupled tothe second reflection port of the first 4-port TF-GFF, wherein thesecond DFG is coupled to the second reflection port of the second 4-portTF-GFF, wherein the first node is arranged at a side of the secondreflection port of the first 4-port TF-GFF, and wherein the second nodeis arranged at a side of the second reflection port of the second 4-portTF-GFF.
 5. The system of claim 4, wherein the first and second 4-portTF-GFFs are configured such that a transmission path insertion loss (IL)targets a gain flattening spectrum.
 6. The system of claim 4, wherein atleast a path connecting the first DFG, the first 4-port TF-GFF, the OTDRfilter, the second 4-port TF-GFF, and the second DFG form a high-lossloop-back (HLLB) path in the optical repeater for the system monitoring.7. The system of claim 1, wherein the optical repeater furthercomprises: a first dual fiber grating (DFG) and a second DFG, andwherein the first DFG is coupled to the second transmission port of thefirst 4-port TF-GFF, wherein the second DFG is coupled to the secondtransmission port of the second 4-port TF-GFF, wherein the first node isarranged at a side of the second transmission port of the first 4-portTF-GFF, and wherein the second node is arranged at a side of the secondtransmission port of the second 4-port TF-GFF.
 8. The system of claim 4,wherein the optical repeater further comprises: a first coupler, asecond coupler, a third coupler, and a fourth coupler, and wherein thefirst coupler is arranged at a side of the first transmission port ofthe first 4-port TF-GFF, wherein the second coupler is arranged at theside of the second reflection port of the first 4-port TF-GFF, whereinthe third coupler is arranged at a side of the first transmission portof the second 4-port TF-GFF, and wherein the fourth coupler is arrangedat the side of the second reflection port of the second 4-port TF-GFF.9. The system of claim 8, wherein the optical repeater furthercomprises: a first path connecting the second and third couplers suchthat the one or more LME channel signals received at the first node arerouted to an input and an output of the optical repeater at the secondfiber; and a second path connecting the first and fourth couplers suchthat the one or more LME channel signals received at the second node arerouted to an input and an output of the optical repeater at the firstfiber, and wherein at least the first path and the second path form along reach high-loss loop-back (HLLB) path in the optical repeater forthe system monitoring.
 10. The system of claim 4, wherein the opticalrepeater further comprises: a first coupler, a second coupler, a thirdcoupler, and a fourth coupler, and wherein the first coupler is arrangedat a side of the first transmission port of the first 4-port TF-GFF,wherein the second coupler is arranged at the side of the secondtransmission port of the first 4-port TF-GFF, wherein the third coupleris arranged at a side of the first transmission port of the second4-port TF-GFF, and wherein the fourth coupler is arranged at the side ofthe second transmission port of the second 4-port TF-GFF.
 11. The systemof claim 10, wherein the optical repeater further comprises: a firstpath connecting the second and third couplers such that the one or moreLME channel signals received at the first node are routed to an inputand an output of the optical repeater at the second fiber; and a secondpath connecting the first and fourth couplers such that the one or moreLME channel signals received at the second node are routed to an inputand an output of the optical repeater at the first fiber, and wherein atleast the first path and the second path form a long reach high-lossloop-back (HLLB) path in the optical repeater for the system monitoring.12. The system of claim 8, wherein the optical repeater furthercomprises: a first path connecting the second and third couplers suchthat the one or more LME channel signals received at the first node arerouted to an input of the optical repeater at the second fiber; and asecond path connecting the first and fourth couplers such that the oneor more LME channel signals received at the second node are routed to aninput of the optical repeater at the first fiber, and wherein at leastthe first path and the second path form a double-pass high-lossloop-back (HLLB) path in the optical repeater for the system monitoring.13. The system of claim 10, wherein the optical repeater furthercomprises: a first path connecting the second and third couplers suchthat the one or more LME channel signals received at the first node arerouted to an input of the optical repeater at the second fiber; and asecond path connecting the first and fourth couplers such that the oneor more LME channel signals received at the second node are routed to aninput of the optical repeater at the first fiber, and wherein at leastthe first path and the second path form a double-pass high-lossloop-back (HLLB) path in the optical repeater for the system monitoring.14. A method for optical signal amplification, comprising: amplifying,via an optical repeater, a first signal input to a first fiber;amplifying, via the optical repeater, a second signal input to a secondfiber; receiving one or more line monitoring equipment (LME) channelsignals from an LME for propagating the one or more LME channel signalsin the optical repeater; and providing the propagated one or more LMEchannel signals to the LME for monitoring the optical repeater, andwherein the optical repeater includes a first 4-port thin-film gainflattening filter (TF-GFF) coupled to the first fiber and a second4-port TF-GFF coupled to the second fiber, and wherein at least thefirst and second 4-port TF-GFFs are connected to form a high-lossloop-back (HLLB) path.
 15. The method of claim 14, wherein the first4-port TF-GFF has a first transmission port, a second transmission port,a first reflection port, and a second reflection port.
 16. The method ofclaim 15, wherein the second 4-port TF-GFF has a first transmissionport, a second transmission port, a first reflection port, and a secondreflection port.
 17. The method of claim 16, wherein the firstreflection port of the first 4-port TF-GFF and the first reflection portof the second 4-port TF-GFF are connected to an optical time-domainreflectometer (OTDR) filter.
 18. An article comprising: a firstdual-fiber collimator, wherein the first dual-fiber collimator has afirst port and a second port; a second dual-fiber collimator, whereinthe second dual-fiber collimator has a third port and a fourth port; anda filter plate arranged between the first dual-fiber collimator and thesecond dual-fiber collimator, and wherein the first port, the secondport, the third port, and the fourth port are configured to receiveand/or transmit light.
 19. The article of claim 18, wherein: (i) thelight input at the first port is partially transmitted through thefilter plate and traced to the fourth port, and the light input at thefirst port is reflected from the filter plate and traced to the secondport, or (ii) the light input at the second port is partiallytransmitted through the filter plate and traced to the third port, andthe light input at the second port is reflected from the filter plateand traced to the first port; or (iii) the light input at the third portis partially transmitted through the filter plate and traced to thesecond port, and the light input at the third port is reflected from thefilter plate and traced to the fourth port; or (iv) the light that isinput at the fourth port is partially transmitted through the filterplate and traced to the first port, and the light that is input at thefourth port is reflected from the filter plate and traced to the thirdport.
 20. The article of claim 18, wherein the article is a 4-portthin-film gain flattening filter (TF-GFF).